1. Field of the Invention
The present invention relates to a photoelectric conversion device. More particularly, the present invention relates to a solar cell that converts sunlight into electric energy.
2. Description of the Related Art
In recent years, production costs of photoelectric conversion devices have been reduced and, as a result, photoelectric conversion layers in the photoelectric conversion devices have been thinned. However, a thinned photoelectric conversion layer transmits therethrough a part of incoming light to result in insufficient light absorption. A photoelectric conversion device including a thinned photoelectric conversion layer therefore has a reduced optical energy conversion efficiency. With such a background, there has been developed a technique to form a structure on a surface of the photoelectric conversion device to enhance light absorption in the photoelectric conversion layer.
For example, a solar cell is known in which numerous rod antennas formed from a dielectric substance are provided on a surface of the solar cell body (see Japanese Unexamined Patent Publication No. 2002-76414). The patent document reports that in the solar cell, electromagnetic waves emitted from the sun, that is, sunlight is received efficiently by the rod antennas and the energy thereof is supplied to the solar cell body.
In addition, a photoelectric device is known in which fine particles are dispersed on an upper surface of a photoelectric conversion layer (quantum well structure, for example), and the fine particles each have a composite structure including a dielectric core and a metal shell portion covering an outer peripheral surface of the dielectric core and produce local surface plasmon resonance with two-wavelength light when receiving light (see Japanese Unexamined Patent Publication No. 2010-21189, for example). The patent document discloses that when light of a wavelength causing the local surface plasmon resonance is delivered to the photoelectric device, a high electric field (near-field light) intensified by the local surface plasmon resonance is generated around the fine particles and, as a result, the probability of the light absorption in the photoelectric conversion layer increases to enhance the conversion efficiency of the photoelectric device.
However, conventional solar cells having such rod antennas cannot localize (or concentrate) sunlight to the rod antennas, because the sunlight is supplied into the solar cell by reradiation. In the case of a thinned photoelectric conversion layer, therefore, the sunlight may be transmitted without being sufficiently absorbed by the photoelectric conversion layer. In addition, production of the rod antennas is costly to be likely to hinder cost reduction (for example, production of a larger rod antenna needs more material).
Besides, in the conventional photoelectric device in which fine particles are dispersed on an upper surface of a photoelectric conversion layer, the electric field is not intensified with wavelengths other than the resonance wavelength for the local surface plasmon resonance and the fine particles may block light to form shadows. That is, the resonance wavelength width for the local surface plasmon resonance is narrow, and therefore intensification of the electric field may be limited to a part of the whole sunlight spectrum. Even when a plurality of resonance wavelengths are generated, for example, each fine particle has only one resonance wavelength and will form a shadow with the other wavelengths. Thus, the local surface plasmon resonance may not sufficiently contribute to enhancement of the efficiency of the photoelectric device. Furthermore, production of the metal shell portion, which is mainly made of a noble metal, of the fine particles may be costly.
With the above-described background, a photoelectric conversion device including a thinned photoelectric conversion layer and having a high conversion efficiency is desired. In addition, a photoelectric conversion device that can be produced at low cost is desired.